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Shi O, Li P, Yang C, Jiang H, Qin L, Liu W, Li X, Chen Z. Melting Behaviors of Bio-Based Poly(propylene 2,5-furan dicarboxylate)-b-poly(ethylene glycol) Co Polymers Related to Their Crystal Morphology. Polymers (Basel) 2023; 16:97. [PMID: 38201762 PMCID: PMC10780312 DOI: 10.3390/polym16010097] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2023] [Revised: 12/15/2023] [Accepted: 12/26/2023] [Indexed: 01/12/2024] Open
Abstract
In this experiment, a series of poly(propylene 2,5-furan dicarboxylate)-b-poly(ethylene glycol) (PPFEG) copolymers with different ratios were synthesized using melt polycondensation of dimethylfuran-2,5-dicarboxylate (DMFD), 1,3-propanediol (PDO) and poly(ethylene glycol) (PEG). The effect of PEG content on the crystallization behavior of the poly(propylene 2,5-furan dicarboxylate) (PPF) copolymers was investigated. For PPF, the nucleation density of the β-crystals was higher than that of α-crystals. As Tc increases, the β crystals are suppressed more, but at Tc = 140 °C, the bulk of PPF has already been converted to α crystals, which crystallize faster at higher nucleation densities, resulting in a difference in polymer properties. For this case, we chose to add a soft segment material, PEG, which led to an early multi-melt crystallization behavior of the PPF. The addition of PEG led to a decrease in the crystallization temperature of PPF, as well as a decrease in the cold crystallization peak of PPF. From the crystalline morphology, it can be seen that the addition of PEG caused the transformation of the PPF crystalline form to occur earlier. From the crystalline morphology of PPF at 155 °C, it can be observed that the ring-banded spherical crystals of the PPF appear slowly with increasing time. With the addition of PEG, spherical crystals of the ring band appeared earlier, and even appeared first, and then disappeared slowly.
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Affiliation(s)
- Ouyang Shi
- School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China; (O.S.); (C.Y.); (W.L.)
| | - Peng Li
- School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China; (O.S.); (C.Y.); (W.L.)
- Guangxi Key Laboratory of Comprehensive Utilization of Calcium Carbonate Resources, College of Materials and Environmental Engineering, Hezhou University, Hezhou 542899, China; (H.J.); (L.Q.)
| | - Chao Yang
- School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China; (O.S.); (C.Y.); (W.L.)
| | - Haitian Jiang
- Guangxi Key Laboratory of Comprehensive Utilization of Calcium Carbonate Resources, College of Materials and Environmental Engineering, Hezhou University, Hezhou 542899, China; (H.J.); (L.Q.)
| | - Liyue Qin
- Guangxi Key Laboratory of Comprehensive Utilization of Calcium Carbonate Resources, College of Materials and Environmental Engineering, Hezhou University, Hezhou 542899, China; (H.J.); (L.Q.)
| | - Wentao Liu
- School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China; (O.S.); (C.Y.); (W.L.)
| | - Xiaolin Li
- Guangxi HuaLong Resin Co., Ltd., Hezhou 542899, China;
| | - Zhenming Chen
- School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China; (O.S.); (C.Y.); (W.L.)
- Guangxi Key Laboratory of Comprehensive Utilization of Calcium Carbonate Resources, College of Materials and Environmental Engineering, Hezhou University, Hezhou 542899, China; (H.J.); (L.Q.)
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Jiang H, Gao J, Zhang X, Guo N. Composite Micro-Nanoarchitectonics of MMT-SiO 2: Space Charge Characteristics under Tensile State. Polymers (Basel) 2021; 13:polym13244354. [PMID: 34960908 PMCID: PMC8707286 DOI: 10.3390/polym13244354] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2021] [Revised: 11/30/2021] [Accepted: 12/03/2021] [Indexed: 11/25/2022] Open
Abstract
Low density polyethylene (LDPE) is a good insulating material which is widely used in cable materials due to its excellent insulation and processability. However, in the DC high voltage environment, pure polyethylene materials still face many problems, the most serious of which is space charge accumulation. The cable will inevitably be subjected to tensile stress during production, installation and operation. Therefore, it is of great significance to study the effect of stretching on the microstructure and space charge characteristics for polymers and their composites. In this paper, MMT/LDPE micro-composites, SiO2/LDPE nano-composites and MMT-SiO2/LDPE micro-nano-composites were prepared by melt blending. Mechanical stretching was carried out on pure LDPE materials and the above three kinds of composite materials. Each material was stretched according to four stretching ratios, which are 0%, 5%, 10% and 20%. The crystal morphology was observed by polarizing microscope (PLM), the crystallization perfection was tested by differential scanning calorimetry (DSC), and the space charge distribution inside each sample was measured by pulsed electro-acoustic (PEA) method. At the same time, the average charge density and apparent charge mobility for samples during depolarization were calculated and analyzed. The experimental results show that when the pure low density polyethylene sample is not stretched, its crystal structure is loose. Tensile stress can make the loose molecular chains align in LDPE and improve its crystalline structure, which is helpful to restrain the accumulation of space charge inside the sample. For MMT/LDPE, SiO2/LDPE and MMT-SiO2/LDPE composites, their internal crystal structure is compact. Stretching will destroy their original crystal structure at first, and then disorder molecular chains inside the three composite materials. With the increase of stretching ratio, the molecular chains begin to orient along the direction of force, the crystallization tends to be perfect gradually, and the space charge accumulation in samples also decreases. From the calculation results of apparent charge mobility for each sample, with the increase of stretching ratio, the trap depth and trap density inside samples firstly increased and then decreased.
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Affiliation(s)
| | | | | | - Ning Guo
- Correspondence: (J.G.); (X.Z.); (N.G.)
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Yu G, Yu B. The Effect of the Cooling Process on the Crystalline Morphology and Dielectric Properties of Polythene. Materials (Basel) 2020; 13:ma13122791. [PMID: 32575741 PMCID: PMC7345052 DOI: 10.3390/ma13122791] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/01/2020] [Revised: 06/13/2020] [Accepted: 06/18/2020] [Indexed: 06/11/2023]
Abstract
In this study, LDPE samples were prepared by melt blending with different cooling processes, which were natural air cooling, rapid air cooling, water cooling and oil cooling, respectively. According to polarization microscope (PLM) and differential scanning calorimeter (DSC) tests of these four low-density polyethylene (LDPE) samples, the effect of different cooling processes on polythene crystalline morphology could be studied. According to conductivity, dielectric frequency spectra and space charge tests, the effect of crystalline morphology on dielectric macroscopic properties could be explored. The microstructure characteristic results indicated the cooling medium significantly affected polythene crystalline morphology. When the samples were produced with natural air cooling, the crystalline grain size was large. On the other hand, after rapid air cooling, water cooling and oil cooling processes, the samples' crystalline grain dispersed uniformly, and the grain sizes were lower. The space charge testing results indicate the samples produced with water cooling and oil cooling processes restrained the electrode injection in the process of pressurization. During short-circuits, the rates of charge release of these two samples were fast, and the remaining space charges were fewer. The conductivity and dielectric frequency spectra testing results indicated the conductivities of samples produced with water cooling and oil cooling processes were both less than those of samples produced with a natural air cooling process. Besides, with increasing experimental frequency, the relative dielectric constants of all testing samples decreased. Among them, the relative dielectric constant of the LDPE sample with the natural air cooling process was the largest. However, the crystalline structures of samples produced with rapid air cooling and water cooling processes were close, which restrained the movement of polymer macromolecule chains. Thus, the dielectric constants were lower. Additionally, because of the influence of relaxation polarization and dipole polarization, the dielectric losses of LDPE with water cooling and oil cooling processes increased to varying degrees.
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Affiliation(s)
- Guang Yu
- Mechanical and Electrical Engineering Institute, University of Electronic Science and Technology of China, Zhongshan Institute, Zhongshan 528400, China
| | - Boyang Yu
- Mechanical and Electrical Engineering Institute, Zhongshan Polytechnic, Zhongshan 528400, China;
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Cheng Y, Yu G, Zhang X, Yu B. The Research of Crystalline Morphology and Breakdown Characteristics of Polymer/Micro-Nano-Composites. Materials (Basel) 2020; 13:E1432. [PMID: 32245191 DOI: 10.3390/ma13061432] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/12/2020] [Revised: 03/15/2020] [Accepted: 03/17/2020] [Indexed: 11/17/2022]
Abstract
In this article, low-density polyethylene (LDPE) was used as a matrix polymer, the Micro-ZnO and Nano-ZnO particles were used as the inorganic filler. With the melt blending method, the Nano-ZnO/LDPE(Nano-ZnO particles doping into LDPE), Micro-ZnO/LDPE(Micro-ZnO particles doping into LDPE) and Micro-Nano-ZnO/LDPE (Nano-ZnO and Micro-ZnO particles doping into LDPE in the same time) composites were prepared. Then, the inorganic filler and the composites were dealt with structural characterizations and analysis by Fourier transform infrared (FTIR), Polarization microscope (PLM), and Differential scanning calorimeter (DSC). Besides, these samples were dealt with (alternating current) AC breakdown performance test. The micro-experimental results showed that the Micro-ZnO and Nano-ZnO particles doping reduced the crystal size and increased the crystallization rate. With the change of cell structure, the crystallinity of composites increased. The crystallinity order of different samples was as follows: LDPE < Micro-ZnO/LDPE < Nano-ZnO/LDPE < Micro-Nano-ZnO/LDPE. From the breakdown of the experimental result, with the same mass fraction of the different inorganic doping of particles, the breakdown strength of these composites was different. The Nano-ZnO particle doping could improve the breakdown strength of composites effectively. Among them, the breakdown strength of Nano-ZnO/LDPE and Micro-Nano-ZnO/LDPE were 11% higher and 1.3% lower than that of pure LDPE, respectively. Meanwhile, the breakdown strength of Micro-composite was the lowest but its Weibull shape coefficient was the highest. Therefore, the Micro-ZnO doping was helpful for the Nano-ZnO dispersing in the matrix, which produced the Micro-Nano-synergy effects better.
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Cheng Y, Yu G. The Research of Interface Microdomain And Corona-Resistance Characteristics of Micro-Nano-ZnO/LDPE. Polymers (Basel) 2020; 12:polym12030563. [PMID: 32143427 PMCID: PMC7182940 DOI: 10.3390/polym12030563] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2020] [Revised: 02/24/2020] [Accepted: 02/28/2020] [Indexed: 11/16/2022] Open
Abstract
In this article, the melting blend was used to prepare the Micro-ZnO/LDPE, Nano-ZnO/LDPE and Micro-Nano-ZnO/LDPE with different inorganic particles contents. The effect of Micro-ZnO and Nano-ZnO particles doping on interface microdomain and corona-resistance breakdown characteristics of LDPE composite could be explored. Based on the energy transfer and heat exchange theory of energetic electrons, the inner electrons energy transfer model of different ZnO/LDPE composites was built. Besides, the microstructure and crystalline morphology of inorganic ZnO-particles and polymer composites were detected by SEM, XRD, FTIR, PLM and DSC test, and the AC breakdown and corona-resistance breakdown characteristics of composites could be explored. From the experimental results, the Nano-ZnO particles after surface modification dispersed uniformly in LDPE matrix, and the nanoparticles agglomeration almost disappeared. The inorganic particles doping acted as the heterogeneous nucleation agent, which improved the crystallization rate and crystallinity of polymer composites effectively. The ZnO particles with different size doping constituted the different interface structure and crystalline morphology, which made different influence on composites macroscopic properties. When the Nano-ZnO particle size was 40nm and the mass fraction was 3%, the breakdown field strength of Nano-ZnO/LPDE was the highest and 15.8% higher than which of pure LDPE. At the same time, the shape parameter β of Micro-Nano-composite was the largest. It illustrated the microparticles doping reduced the probability of nanoparticles agglomeration in matrix. Besides, both Micro-ZnO and Micro-Nano-ZnO particles doping could improve the ability of corona corrosion resistance of LDPE in varying degrees. The corona-resistant breakdown time order of four samples was as follows: LDPE < Micro-ZnO/LDPE < Nano-ZnO/LDPE < Micro-Nano-ZnO/LDPE. When the mass fraction of Micro-ZnO and Nano-ZnO particles was 2% and 3% respectively, the corrosion depth and area of Micro-Nano-ZnO/LDPE was the least, and the ability of corona corrosion resistance was the strongest.
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Affiliation(s)
| | - Guang Yu
- Correspondence: ; Tel./Fax: +86-0760-8826-9835
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Muñoz-Pascual S, Lopez-Gonzalez E, Saiz-Arroyo C, Rodriguez-Perez MA. Effect of Mold Temperature on the Impact Behavior and Morphology of Injection Molded Foams Based on Polypropylene Polyethylene⁻Octene Copolymer Blends. Polymers (Basel) 2019; 11:polym11050894. [PMID: 31096687 PMCID: PMC6572697 DOI: 10.3390/polym11050894] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2019] [Revised: 05/09/2019] [Accepted: 05/10/2019] [Indexed: 12/01/2022] Open
Abstract
In this work, an isotactic polypropylene (PP) and a polyethylene–octene copolymer (POE) have been blended and injection-molded, obtaining solids and foamed samples with a relative density of 0.76. Different mold temperature and injection temperature were used. The Izod impact strength was measured. For solids, higher mold temperature increased the impact resistance, whereas in foams, the opposite trend was observed. In order to understand the reasons of this behavior, the morphology of the elastomeric phase, the crystalline morphology and the cellular structure have been studied. The presence of the elastomer near the skin in the case of high mold temperature can explain the improvement produced with a high mold temperature in solids. For foams, aspects as the elastomer coarsening in the core of the sample or the presence of a thicker solid skin are the critical parameters that justify the improved behavior of the materials produced with a lower mold temperature.
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Affiliation(s)
| | - Eduardo Lopez-Gonzalez
- Cellular Laboratory (CellMat), Universidad de Valladolid, 47011 Valladolid, Spain.
- CellMat Technologies S.L., Paseo de Belen 9-A (CTTA Building), 47011 Valladolid, Spain.
| | - Cristina Saiz-Arroyo
- CellMat Technologies S.L., Paseo de Belen 9-A (CTTA Building), 47011 Valladolid, Spain.
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Yang J, Gao M, Zhao H, Liu S, Hu M, Xie S. Space Charge Characteristics of Polypropylene Modified by Rare Earth Nucleating Agent for β Crystallization. Materials (Basel) 2018; 12:E42. [PMID: 30586851 DOI: 10.3390/ma12010042] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/04/2018] [Revised: 12/18/2018] [Accepted: 12/20/2018] [Indexed: 11/17/2022]
Abstract
Compared to cross-linked polyethylene, polypropylene has a thermoplastic property and the advantage of recycling. However, the poor impact resistance at low temperature and the corresponding space charge problem restrict the application of polypropylene with the extruded high voltage direct current (HVDC) cable. Sufficient introduction of the β form of the polypropylene crystal can significantly improve impact resistance at low temperatures. Although it has been widely applied in insulation engineering, the effect of β-crystal on the space charge characteristics of polypropylene has rarely been researched until now. In this paper, a rare earth nucleating agent of β-crystal is employed to modify the performance of polypropylene to investigate the effects of nucleating agent content on β-crystalline, mechanical relaxation, trap, and space charge characteristics of polypropylene. The results of differential scanning calorimeter (DSC) and X-ray diffraction (XRD) tests indicate that the relative content of β-crystal in modified polypropylene increases gradually with the increasing concentration of the nucleating agent, approaching 43.5% when the nucleating agent content has been raised to 0.2 wt %, suggesting appreciable efficiency of the nucleating agent utilized in our research. Scanning electron microscopy (SEM) is utilized to characterize the morphology of β-crystal spherulites, which illustrates that the β-spherulites are in bunchy shape, and the lamellar crystals are parallel to each other without an obvious boundary between them. The results of the space charge test demonstrate that the modified polypropylene can substantially suppress space charge accumulation, which is attributed to an increment of β-crystal content by adopting a rare earth nucleating agent. It is indicated from dynamic mechanical analysis (DMA) measurements that the enhancement of β-crystalline in modified polypropylene can distinctly increase and decrease the β and α relaxation losses, respectively, which proves that the defects in β-crystal and amorphous regions are reduced and increased respectively. Thermally stimulated depolarization current tests further confirm that the number of traps caused by defects in the β-form of polypropylene crystal declines definitely, which dominantly accounts for the suppression of space charge accumulation.
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Kuzmanović M, Delva L, Mi D, Martins CI, Cardon L, Ragaert K. Development of Crystalline Morphology and Its Relationship with Mechanical Properties of PP/PET Microfibrillar Composites Containing POE and POE- g-MA. Polymers (Basel) 2018; 10:E291. [PMID: 30966326 PMCID: PMC6414988 DOI: 10.3390/polym10030291] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2018] [Revised: 03/02/2018] [Accepted: 03/05/2018] [Indexed: 11/17/2022] Open
Abstract
The main goal of this research is to study the development of crystalline morphology and compare it to various mechanical properties of microfibrillar composites (MFCs) based on polypropylene (PP) and poly(ethylene terephthalate) (PET), by adding a functional compatibilizer and a non-functional rubber in two different steps in the processing sequence. The MFCs were prepared at a weight ratio of 80/20 PP/PET by twin screw extrusion followed by cold drawing and injection moulding. The non-functionalized polyolefin-based elastomer (POE) and the functional compatibilizer (i.e., POE grafted with maleic anhydride (POE-g-MA)) were added in a fixed weight percentage at two stages: during extrusion or during injection moulding. The morphology observations showed differences in crystalline structure, and the PP spherulite size was reduced in all MFCs due to the presence of PET fibrils. Their relationship with the mechanical performances of the composite was studied by tensile and impact tests. Adding the functional compatibilizer during extrusions showed better mechanical properties compared to MFCs. Overall, a clear relationship was identified between processing, structure and properties.
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Affiliation(s)
- Maja Kuzmanović
- Centre for Polymer and Material Technologies, Department of Materials, Textiles and Chemical Engineering, Faculty of Engineering and Architecture, Ghent University, Technologiepark 915, Zwijnaarde 9052, Belgium.
| | - Laurens Delva
- Centre for Polymer and Material Technologies, Department of Materials, Textiles and Chemical Engineering, Faculty of Engineering and Architecture, Ghent University, Technologiepark 915, Zwijnaarde 9052, Belgium.
| | - Dashan Mi
- Centre for Polymer and Material Technologies, Department of Materials, Textiles and Chemical Engineering, Faculty of Engineering and Architecture, Ghent University, Technologiepark 915, Zwijnaarde 9052, Belgium.
- College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China.
| | - Carla Isabel Martins
- Institute for Polymer and Composites/I3N, University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal.
| | - Ludwig Cardon
- Centre for Polymer and Material Technologies, Department of Materials, Textiles and Chemical Engineering, Faculty of Engineering and Architecture, Ghent University, Technologiepark 915, Zwijnaarde 9052, Belgium.
| | - Kim Ragaert
- Centre for Polymer and Material Technologies, Department of Materials, Textiles and Chemical Engineering, Faculty of Engineering and Architecture, Ghent University, Technologiepark 915, Zwijnaarde 9052, Belgium.
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Ko EY, Park GE, Lee DH, Um HA, Shin J, Cho MJ, Choi DH. Enhanced Performance of Polymer Solar Cells Comprising Diketopyrrolopyrrole-Based Regular Terpolymer Bearing Two Different π-Extended Donor Units. ACS Appl Mater Interfaces 2015; 7:28303-28310. [PMID: 26645584 DOI: 10.1021/acsami.5b08510] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
New regular and random diketopyrrolopyrrole (DPP)-based terpolymers (i.e., Reg-PBDPPT and Ran-PBDPPT, respectively) bearing DPP as an electron deficient unit and 2,2'-bithiophene and (E)-1,2-di(thiophen-2-yl)ethene as electron donating units were designed and synthesized, and their performance in photovoltaic cells was investigated precisely. The absorption properties and highest occupied molecular orbital (HOMO) of Reg-PBDPPT were found to be different from those of Ran-PBDPPT. The results of grazing incidence X-ray diffraction experiments revealed that Ran-PBDPPT typically had a predominantly edge-on chain orientation on the substrate, whereas Reg-PBDPPT showed mixed chain orientation both in pristine and thermally annealed films. Although Reg-PBDPPT exhibited a lower degree of edge-on chain orientation on the substrate, the corresponding TFTs showed a high hole mobility of 0.42-0.96 cm(2) V(-1) s(-1) and maintained a high current on/off ratio (>10(6)). A polymer solar cell (PSC) composed of Reg-PBDPPT and PC71BM exhibited power conversion efficiencies (PCE) of 5.24-5.45%, which were higher than those of the Ran-PBDPPT-based PSCs. The enhanced efficiency was supported by an increase in the short circuit current, which is strongly related to the unique internal crystalline morphology and pronounced nanophase segregation behavior in the blend films. These results obviously manifested that this synthetic strategy for regular conjugated terpolymers could be employed to control morphological properties to obtain high-performance PSCs.
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Affiliation(s)
- Eun Yi Ko
- Department of Chemistry, Research Institute for Natural Sciences Korea University , 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
| | - Gi Eun Park
- Department of Chemistry, Research Institute for Natural Sciences Korea University , 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
| | - Dae Hee Lee
- Department of Chemistry, Research Institute for Natural Sciences Korea University , 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
| | - Hyun Ah Um
- Department of Chemistry, Research Institute for Natural Sciences Korea University , 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
| | - Jicheol Shin
- Department of Chemistry, Research Institute for Natural Sciences Korea University , 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
| | - Min Ju Cho
- Department of Chemistry, Research Institute for Natural Sciences Korea University , 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
| | - Dong Hoon Choi
- Department of Chemistry, Research Institute for Natural Sciences Korea University , 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
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